In a number of industrial and municipal applications, such as wastewater treatment and desalination of sea water, membrane-based filtration processes, particularly crossflow filtration, have been used for decades. In the latter process, a liquid to be purified—also referred to as the feed—flows tangentially to the membrane surface over porous membranes in planar form. Depending on the application, the pore size of the membranes is in the range of a few nanometers to a few micrometers. The volume through which the feed flows, commonly referred to as the feed space, is separated by the membrane from a permeate space. A differential pressure of approximately 0.1 bar to 100 bar is applied between the feed space and the permeate space, which causes material to be transported from the feed space to the permeate space, causing permeate (or filtrate) to enter the permeate space. Membrane bioreactors (MBR) used in wastewater treatment are preferably operated with a pressure differential in the range of 0.02 to 0.4 bar.
In the MBR process, the wastewater is physically, chemically, and biologically treated in several steps until it reaches the filtration membrane. Particles, fibers, and coarse matter are removed from the wastewater by mechanical and physical pretreatment. In coarse filtration, large particles that could cause damage to the membranes are removed by grills and screens. Following this, fine screens in a size range of 0.05-3 mm are commonly used for prefiltration. Heavy particles (e.g., sand), oils, and fats are also removed from the wastewater by a sand and fat trap.
In a further treatment step, the wastewater is biologically and chemically treated. An activation tank contains sludge (biomass) with microorganisms that enzymatically convert and eliminate the high-molecular-weight organic contaminants. The residual materials following enzymatic conversion are used by the microorganisms either for cell building or energy production while consuming oxygen. The resulting oxygen consumption must be offset by a sufficient oxygen supply, for which purpose activating tanks are provided with aeration systems. The prerequisite for functioning of the process is that the biomass remain in the system. The biomass is therefore separated from the purified wastewater by membrane filtration and fed back to the activation tank. Built-up activated sludge is removed as surplus sludge. Before the biomass is separated from the water, further chemical treatments are carried out as needed. For this purpose, various precipitants and flocculants, such as iron chloride or polymers, are commonly used in combination with a filtration step in order to remove colloidal and particulate matter dissolved in the liquid.
The solid-free effluent is an essential advantage of MBR units. The effluent from an MBR unit contains no bacteria; even viruses are often separated out by sorption effects. This sharply reduces residual organic contamination. The MBR process is compliant with the hygienically relevant guideline values of the EU Bathing Water Directive [75/160/EEC, 2006]. Moreover, the solid-free effluent offers great potential for wastewater reuse in both the industrial and municipal sectors. This allows significant water conservation to be achieved using methods from water recycling to water circuit closure. Another advantage of the MBR process is that it requires far less space than the conventional activation process. The MBR modules replace secondary clarification, which in the conventional process is carried out in large secondary clarifiers in which the biomass precipitates. For this purpose, the MBR modules are submerged in the activation tank or used in separated filter chambers. In addition to obviating the need for secondary clarifiers, the footprint can be further reduced because the independence of the sedimentation process allows the activated sludge concentration (biomass concentration, expressed as DM—dry matter) to be increased compared to conventional processes. Membrane bioreactors are ordinarily operated with DM concentrations of 8 to 15 g/L, higher by a factor of 2-3 than conventional processes. The reactor volume in the MBR process can be reduced compared to conventional activation processes, making higher volumetric loading rates possible.
Filtration membranes are known from prior art. Some of the known filtration membranes are formed as a two-layer composite of a supporting nonwoven and a porous membrane layer. The porous membrane layer preferably consists of polyethersulfone, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyamide, polyetherimide, cellulose acetate, regenerated cellulose, polyolefins, or fluoropolymers. For example, the porous membrane layer may be produced, for example, by coating a nonwoven or fabric with polymer solution and precipitating the polymer in a subsequent phase inversion step. Alternatively, a polymer film may be stretched in a suitable manner so that pores are generated in the polymer film. The stretched polymer film is then laminated onto a supporting nonwoven for mechanical stabilization. Filtration membranes produced according to these methods are commercially available, e.g. under the name NADIR® membranes (MICRODYN-NADIR GmbH, Wiesbaden) or CELGARD® Flat Sheet Membranes (Celgard Inc., Charlotte, N.C., USA).
Components contained in the feed whose diameter is too large to pass through the membrane pores, are retained on the membrane surface, where some of them adhere. In crossflow filtration, feed permanently flows over the membrane surface in order to transport the retained components (retentate) away from the membrane surface. This makes continuous filtration operation with constant permeate flow possible. The crossflow mode of operation results in the typical structure of membrane modules, with three connections or passages for feed, retentate, and permeate. Membrane modules are equipped with a housing or frame which is open on one or more sides, and in which flat filter elements, or in rare cases, wound filters are mounted. Depending on the structure, in addition to passages between the filter elements or passages between the windings of the wound filter, a membrane module may optionally have connections for feed, retentate, and permeate arranged on the walls of the housing.
In a flat filter element, the permeate space is bounded by two separate membranes or two partial surfaces of a one-piece membrane. A porous permeate spacer is arranged between the two membranes or partial surfaces that on the one hand acts as a supporting structure for the sensitive membranes, which are subjected to a transmembrane differential pressure of up to 100 bar, and on the other provides passages through which the permeate flows off along the inner side of the membrane/partial surface. In a membrane module having several flat filter elements, the permeate space is composed of all of the permeate spaces of all flat filter elements.
In flat filter modules, a plurality of planar flat filter elements are arranged in parallel in a stack. Spacers are arranged between each two adjacent flat filter elements that keep a passage open through which the feed and retentate can flow in and out. The spacers comprise, for example, washers made of a polymeric material that are arranged between the peripheral areas or edges, and particularly the corners, of each two adjacent flat filter elements. Alternatively, a frame or housing equipped with equidistant grooves for holding the edges of the flat filter elements can be used.
Filtration membranes suitable for MBR have a cutoff of less than 400 nm and an operating permeability of more than 100 L/(m2·h≤bar), and preferably more than L/(m2·h·bar. The cutoff refers to the diameter of the smallest particles retained by the membrane. Because of surface layer formation during filtration, the effective cutoff is sharply lower, so that even most viruses in the area of 25 nm are retained.
According to a highly simplified model concept, a filtration membrane consists of a solid material penetrated by a plurality of cylindrical pores that are oriented perpendicularly to the surface of the filtration membrane, wherein all of the pores are of the same diameter. In this simple model, the cutoff corresponds to the diameter of the cylindrical pores. Real filtration membranes show a complex morphology, with irregular, three-dimensionally branched or spiral-shaped pores or passages. The pores of actual filtration membranes have an area of minimal diameter that determines the cutoff of the respective pore. In the ideal case, the area of the smallest pore diameter is at the surface of the filtration membrane, so that no particles with a diameter greater than the cutoff can penetrate the pores and plug them. In actual filtration membranes, the area of the minimum pore diameter is at a distance from the membrane surface that varies from pore to pore.
Filtration membranes used for the MBR process have a highly asymmetric structure with a finely porous separating layer 0.5 to 1.0 μm thick and a coarsely porous supporting layer 30 to 100 μm thick. The pore diameters of the separating layer are smaller than 0.1 μm. The finger-like pores of the supporting layer are often referred to in the art as caverns and have a diameter of up to 20 μm. The double-layer structure of these known filtration membranes ensures a cutoff in the range of 0.03 to 0.1 μm together with favorable clean water permeability from 200 to 1000 L/(m2·h·bar). These membranes are provided with a stabilizing agent in the production process. The stabilizing agent prevents the pores in the thin, finely porous separating layer from collapsing when the membrane dries. Collapse of the pores is caused by the enormous capillary forces generated when water evaporates from the small pores and is irreversible. After washing out the stabilizing agent, e.g. after the membranes are put into operation, they must not be dried, as this would cause an irreversible decrease in clean water permeability to less than 10 L/(m2·h·bar).
A problem in the use of membrane filters in the area of wastewater treatment is what is referred to as “membrane fouling”, which is characterized by the formation of deposits on the membrane that reduce permeation and thus filtration performance to values of 50 to 200 L/(m2·h·bar).
Various methods are used to control fouling in the MBR process:                (a) Pretreatment of the untreated water or wastewater before it flows into the activated sludge by means of various filtration steps mentioned above, in which fine-mesh gratings having a mesh size of 0.5 to 3 mm are used;        (b) In the “cross-flow” process, the liquid to be purified is circulated along the membrane surface, and for this purpose, in the case of submerged modules, aeration devices that induce upward flow are installed below the membrane modules;        (c) Regular backwashing with permeate, so that adhering particles/contaminants are detached from the membrane surface and the pores are flushed open. A precondition is that it must be possible to backwash the respective membrane;        (d) Chemical cleaning, wherein the filtration membranes are taken out of service and additional membranes must be installed if necessary. The chemicals used in chemical cleaning, such as sodium hypochlorite NaOCl pollute the environment and form absorbable organic halogen compounds (AOX). In addition, chemical cleaning requires an additional infrastructure (pumps, chemical containers, leak detectors, protective equipment) which is costly. Often, the membranes are cleaned in a separate cleaning container having a small volume in order to reduce the amount of cleaning chemicals. For this purpose, the membrane module must be removed from the filtration tank and transferred to the cleaning container. Chemical cleaning involves considerable cost and adversely affects the environment.        (e) Addition of cleaning granules to the activated sludge, as described for example in the publication by the firm VA Tech Wabag GmbH, Vienna, author: F. Klegraf, entitled “Managing of Fouling and Scaling on Submerged Filtration Systems in Activated Sludge Membrane Units” and in patent application DE 10 2008 021 190 A1. In this case, granules having varying abrasive strengths are used, with said granules being entrained in the crossflow and along the surface of the membranes. Expanded clay and polymers are some of the materials that have been tried out as cleaning granules. The cleaning granules are retained by screens in the filter chambers of the MBR unit. The turbulence introduced into the MBR with the flushing air is sufficient to homogeneously distribute the cleaning granules. Shortly after the MBR unit is charged with cleaning granules, filtration performance increases, and by increasing the concentration of the cleaning granules in the activated sludge, filtration performance can after a certain period be regenerated to 75% of the starting value. Any further increase in the concentration of cleaning granules does not noticeably improve filtration performance. The use of cleaning granules is controversial, as the sensitive surfaces of the filtration membranes are damaged.        
As the separation layer in known asymmetric filtration membranes is only 0.5 to 1.0 μm thick, they are extremely sensitive to the abrasive effect of cleaning granules. In abrasion experiments conducted to investigate this, massive damage and penetration of the separating layer was observed. The clean water permeability values of these membranes increase from 500-1000 L/(m2·h·bar) to 10.000-50.000 L/(m2·h·bar). The mean pore diameter of these membranes increases from 0.03-0.05 μm to 1-10 μm. If the separating layer breaks and the caverns of the coarsely-porous supporting layer are opened, there is a risk that activated sludge will penetrate into the membrane structure and clog large areas of the surface, sharply decreasing filtration performance. In the worst-case scenario, the MBR unit has to be shut down and fitted with new membrane modules.
In addition to the above-mentioned highly asymmetric filtration membranes with a thin separating layer followed by a separating layer with extremely coarse pores, separation membranes with a graded pore profile are known in the art. Such membranes are characterized by a cutoff in the range of 0.05 to 0.4 μm and favorable abrasion resistance to cleaning granules. These membranes can also be dried without using a stabilizing agent because of their larger pores and pore structure. However, such membranes have a low clean water permeability of less than 150 L/(m2·h·bar) and a correspondingly reduced operating permeability of 50 to 100 L/(m2·h·bar), making economical operation of an MBR impossible.